The invention relates to the field of optical measurement using a rapid scanning optical delay line and more specifically to the field of optical coherence tomography.
For many applications in optical ranging and optical imaging using interferometric based techniques, it is necessary to use a scanning optical delay line as a component of the measurement apparatus. A conventional scanning optical delay line produces a delay by propagating the optical beam through a variable path length. Such a conventional delay line produces a change in phase delay and group delay which is determined by the geometric path length divided, respectively, by the phase velocity and group velocity of light in the medium of propagation.
Previous optical delay scanning devices have largely relied on scanning of the optical path length in order to achieve delay scanning. Devices using linear actuators, spinning mirrors or cam-driven linear slides have been demonstrated. Most current mechanical scanning optical delay lines are not rapid enough to allow in vivo imaging owing to the presence of motion artifacts. Piezoelectric optical fiber stretchers that allow rapid scanning have been demonstrated but they suffer from high power requirements, nonlinear fringe modulation due to hysteresis and drift, uncompensated dispersion mismatches, and poor mechanical and temperature stability. In addition the concept of using a system of diffraction gratings and lenses has been demonstrated for stretching and compressing short optical pulses, pulse shaping and phase control. A combination grating and lens device has been demonstrated for scanning delay in a short pulse autocorrelator. The device produces a change in group delay by angular adjustment of a mirror, however, it does not permit the phase delay to be adjusted independently of the group delay.
Such delay lines are useful in performing Optical Coherence Tomography (OCT). OCT is a relatively new optical imaging technique that uses low coherence interferometry to perform high resolution ranging and cross sectional imaging by illuminating the object to be imaged with low coherence light and measuring the back reflected or back scattered light as a function of time delay or range. Optical ranging and imaging in tissue is frequently performed using a modified Michelson or other type interferometer. Precision measurement of optical range is possible since interference is only observed when the optical path length to the scattering features within the specimen and the reference path optical path length match to within the coherence length of the light.
The axial reflectance of structures within the specimen is typically obtained by varying the reference arm length using a mechanical scanning linear galvanometer translator and digitizing the magnitude of the demodulated interference envelope or direct digitization of the fringes. A cross-sectional image is produced by recording axial reflectance profiles while the position of the optical beam on the sample to be imaged is scanned. Such imaging can be performed through various optical delivery systems such as a microscope, hand-held probe, catheter, endoscope, or laparoscope.
Unlike conventional scanning optical delay lines, the change in phase delay using a grating based phase controlled delay line is more independently adjustable from the change in group delay, so that when the delay line is used in conjunction with an interferometer, the modulation of interference fringes produced by delay line scanning may be more precisely controlled. In one embodiment a diffraction grating disperses an optical beam into different spectral frequency or wavelength components which are collimated by a lens. A mirror is placed one focal length away from the lens and the alteration of the grating groove density, the grating input angle, the grating output angle, or the mirror tilt produces a change in optical group and phase delay. Specifically, if the mirror tilt produces a change in group delay, the offset of the beam with respect to the center axis of tilt controls the phase delay and the resultant modulation frequency at the interferometer. Moreover, if the grating-lens pair is incident on the center axis of the tilting mirror, group delay is produced without changing the phase delay. Then other external modulation techniques may be applied to control the frequency of modulation of the interference fringes, or OCT detection can be performed directly at baseband using a phase diversity homodyne detection technique.
In the preferred embodiment, the device permits optical delays to be scanned by scanning an angle of a mirror, thus providing higher speed optical delay scanning than conventional optical delay lines which typically require longitudinal or range scanning of mirrors or other optical retroreflecting elements. In other embodiments, the device permits high speed scanning by varying the periodicity of an acousto-optically generated diffraction grating or other device parameters. In addition, since interferometric optical ranging and imaging techniques depend upon the frequency of modulation of the interference fringes produced by the interferometer, this device permits the design of higher performance interferometric ranging and imaging systems.
The optical delay line apparatus is designed so that it may be used with Low Coherence Interferometry (LCI), Optical Coherence Tomography (OCT), or other interferometric based optical ranging and imaging techniques. This apparatus is especially useful for the implementation of OCT in applications which require high speed imaging because these applications require high speed scanning of optical delay. In medical imaging or in vivo imaging applications, the apparatus permits high speed imaging by reducing or eliminating blurring from motion artifacts and permitting real time visualization. The medical applications of this device in OCT imaging include but are not limited to in vivo medical diagnostic imaging of the vascular system; gastrointestinal tract; urinary tract; respiratory tract; nervous system; embryonic tissue; OB/GYN tissue; and any other internal human organ systems. Other medical applications include a rapid scanning OCT system for performing guiding surgical intervention. This device may be also used in OCT imaging for non-medical applications including imaging in biological specimens, materials, composite materials, semiconductors, semiconductor devices and packages, and other applications requiring high speed imaging.
The optical delay lines of the invention presented here are an improvement over existing mechanical delay lines because the sweep speed of the scan can be increased and the phase delay and group delay of the scanning can be more independently controlled. This decoupling of group delay and phase delay permits the control of fringe modulation in a manner not previously possible by other optical delay scanning methods. Additionally, the disclosed delay scheme can be embodied with no moving parts. Finally, this optical delay line apparatus can be incorporated into OCT systems to enable high speed reference arm optical path length scanning using heterodyne or homodyne detection. This scanning technology is necessary for high speed OCT imaging to for a variety of applications (e.g., in vivo medical imaging in human tissue). It has been shown that OCT has ten times greater resolution than intravascular ultrasound (IVUS) and endoscopic ultrasound (EUS) in the application of diagnosing tissue pathology. Similar findings have shown that OCT may be clinically useful for performing high resolution imaging of other organ systems, including the skin and gastrointestinal tract.
The delay line includes common optical components, has modest power requirements, generates repeatable and controllable optical delays, and is temperature stable. Moreover, since the phase delay and group delay are adjustable, the modulation frequency which is produced in interferometric imaging techniques can be controlled thus simplifying the detection electronics. This is especially important for detection scenarios which involve direct electronic digitization (A/D conversion) of the detected optical interference signal.
The grating based phase control optical delay line produces optical group and phase delay by dispersing the spectrum with a grating, and applying a temporally modulated linear wavelength dependent phase. The linear wavelength dependent phase can be achieved by reflecting the spread spectrum from a tilted mirror. If the angle of the mirror is rapidly scanned, a time dependent optical group delay line is produced. The optical delay line can then be inserted into the reference arm of an interferometer for performing high speed OCT.
The phase control delay line is powerful because it allows group delay to be produced by scanning the angle of a beam, instead of employing mechanical linear translation to vary optical path length. The phase control delay line also allows flexibility in the heterodyne or IF beat frequency. Commercially available mechanical beam scanners such as the galvanometer, resonant scanner, rotating polygon mirror, and scanning holographic optical elements are one to two orders of magnitude faster than mechanical linear translators. In addition, rapid optical beam scanning can be performed by devices such as acousto-optic modulators which contain no moving parts. These components are used in a variety of applications such as bar code readers, laser printers, and real time video scanning subsystems.